FLIP SIDE: Alex Rodriguez tosses his bat after striking out in yesterday’s 3-1 loss to the Cubs.UPI

CHICAGO — It never fails, right? Near the end of an afternoon spent flailing at Doug Davis’ Bugs Bunny slowballs, following inning after inning of popping out meekly and grounding out softly and staring in disbelief as baseballs eased past their knees and slipped over corners, the Yankees were in business. They were in play.

Nick Swisher and Mark Teixeira had hit two of the hardest balls the Yankees had mustered all day in the top of the eighth inning — Swisher doubling off the ivy, Teixeira nearly parting reliever Sean Marshall’s hair with a single up the middle. After two hours of inertia, the Yankees were alive. It was 3-1, Cubs. The tying run was at the plate.

And it was Alex Rodriguez.

That never seems to fail, either. Somehow, the game figures out a way to find Rodriguez, figures a way to make him a key component for good, for bad, for something. It’s who he is. It’s who he always has been. And it was just right that it should happen on this day, the first time Rodriguez wore a Yankees uniform in Wrigley Field.

“It was a good day and a great game,” said A-Rod, a student of the game who loved wearing that uniform in this building. “And we hung in there.”

Closer Carlos Marmol walked in from the Cubs’ bullpen down the left-field line. A few weeks ago, Marmol had let a game get away against the Cardinals, had heard about it from his teammate Carlos Zambrano, but for the Cardinals, facing Marmol is like the Yankees facing Jonathan Papelbon. Familiarity strips a lot of the mystery from a pitcher’s arm.

“I think the more you face him, the more you may get comfortable against him,” Rodriguez said. “But I’d never faced him before.”

Rodriguez didn’t look comfortable on the first two pitches Marmol threw, both sliders, one that bent outside and another that dipped inside. A-Rod flinched at both, didn’t offer, took both for balls. Hitter’s count now.

There were 42,219 people shoehorned into Wrigley, the Cubs’ largest crowd of the year, and with each ball you could hear just how many Yankees fans had finagled a way inside the cozy grounds.

Rodriguez looked less comfortable with the next two.

“On the black,” he said. “Or better.”

Now you heard from the Cubs fans. It has been a tough year for the Cubs so far — hell, it’s been a tough century — but they had just taken three out of four from one first-place team, the NL Central-leading Brewers, and they had cobbled enough early offense to nick three runs off Freddy Garcia before he settled down and retired 14 of the final 15 to face him.

Beyond the outfield fences, the stands that rise up from eight different apartment buildings were mostly full. The last time the Yankees had played here, in 2003, Roger Clemens had been foiled in an attempt for his 300th win, but that Cubs team was destined for a far more meaningful — and heartbreaking — date with history later that season.

So they were ready for these three games with the Yankees. They came pouring out of the neighborhood bars in time to see Davis handcuff the Yankees, they sang along with Seth Meyers during the seventh-inning stretch. The best T-shirt available outside went something like this:

Q: What did Jesus tell Cubs fans?

A: Don’t do anything till I get back.

From here, it sure seems like the Yankees have an unimpeded path toward a 16th playoff berth across the last 17 years, and it seems like the Cubs will muddle through a 103rd straight season without a championship. But for now, it was 2-and-2 on A-Rod, everyone in the joint was up, on their feet. This is why it’s right that these interleague games prosper, for moments like Alex Rodriguez at Wrigley Field, when one swing of the bat can change everything …